1. Field of the Invention
The present invention relates to an organic electroluminescence device (hereinafter sometimes referred to also as an organic EL device) having improved emission characteristics and lifetime characteristics.
2. Discussion of Background
Along with the rapid progress in the technical developments in the information communication field in recent years, more expectations are placed on flat display devices as substitutes for CRT. Among them, an organic EL device is excellent in the high speed response, visibility, luminance, etc., and various studies are being made thereon.
In an organic EL device, electron holes are injected from an anode, and electrons are injected from a cathode, whereby they will be recombined in a light-emitting layer to form excitons. Their energy will be transferred to a light-emitting material to emit light. In a case where the light-emitting layer is constituted by a host compound and a guest compound which is a light-emitting dye, excitons will be formed in the host compound, and they will be transferred to the guest compound, whereupon luminescence of the light-emitting dye can be obtained. With respect to such a combination of a host compound and a guest compound, various combinations of dyes are known for high fluorescent quantum yields or high phosphorescent quantum yields.
With respect to an organic EL device, it is known that the luminance is likely to decrease as the emission time passes. Accordingly, it is important to suppress such a luminance decreasing rate and to improve the lifetime of the device. Further, the organic EL device is usually a device constituted by thin layers, whereby short circuiting between the anode layer and the cathode layer, or a leakage current, is likely to bring about a display defect or shortening of the lifetime.
As a method for improving the lifetime of a device, studies have been made on a method of using a material having high heat resistance for the hole transport layer or a method of doping a light-emitting dye having a long lifetime (such as rubrene) as a guest compound to the light-emitting layer. Further, a method has also been developed in which a plurality of guest compounds are incorporated in a host compound in a light-emitting layer to attain high luminous efficiency or to prolong the lifetime (JP-A-9-134786, JP-A-2000-106277 and JP-A-2002-38140). However, among devices having high luminous efficiency, there are some, of which the half luminance lifetime is short, or some whereby the voltage increase is large during constant current driving.
Further, as a method for suppressing short circuiting, it is known to employ a polymer compound on the surface of the anode layer (J. Kido, Appl. Phys. Lett., 761 (1992)). However, even by using this method, no adequate extension of the lifetime can be attained, and a substantial improvement is still desired.
One of the causes for the large increase of the voltage and the quick decrease of the luminance, is considered to be accumulation of a carrier on the host compound in the light-emitting layer and the low heat resistance of the light-emitting layer. For example, in a case where aluminum tris(8-quinolinolate) (hereinafter referred to as Alq) is used as the host compound in a light-emitting layer, Alq has an electron transporting property which is superior to the hole transporting property (electron mobility: 5.0×10−5 cm2/Vs, hole mobility: 8.0×10−8 cm2/Vs), whereby an injected hole is likely to be retained on one Alq molecule, which leads to degradation of the host compound. This is considered to be a cause for the increase of the driving voltage or the decrease of the luminance.
The above-mentioned degradation of the host compound is not limited to Alq, and similar degradation tends to occur with other host compounds and is considered to be a cause for an increase of the driving voltage or for a decrease of the luminance. Accordingly, it is considered possible to improve the half luminance lifetime and to suppress the increase of the driving voltage, by suppressing retention and accumulation of the carrier which causes degradation of the host compound.
Further, the reason as to why the conventional method of suppressing short circuiting or leakage current by forming a thin film layer of a polymer compound on the surface of the anode layer, did not sufficiently contribute to prolongation of the lifetime of the device, is considered to be such that no adequate consideration is given to the injection or transporting characteristics of the carrier in the after-mentioned hole transport layer or the anode interface layer. In an organic EL device, a hole transport layer is formed between the anode layer and the light-emitting layer in many cases. Further, a thin film layer so-called an anode interface layer (or an anode buffer layer) may sometimes be formed between the hole transport layer and the anode layer. Accordingly, it is conceivable to accomplish prolongation of the lifetime by using a polymer compound having a high covering property and being capable of suppressing the degradation of the light-emitting layer, for such a hole transport layer or an anode interface layer.